1. Technical Field
The present invention relates to a positioning system which uses signals from positioning satellites, an information supply apparatus, a terminal apparatus, a control method of the terminal apparatus, a control program for the terminal apparatus, and a computer readable recording medium for storing the control program for the terminal apparatus.
2. Related Art
Conventionally, positioning systems for positioning a current position of each GPS (Global Positioning System) receiver using a satellite navigation system such as GPS have been used.
Such GPS receiver calculates, for example, the distance (hereinafter, referred to as pseudo range) between each GPS satellite and the GPS receiver based on the difference (hereinafter, referred to as delay time) between times when a signal is transmitted from each GPS receiver and the signal is received by the GPS receiver based on signals from four observable GPS satellites at the current time. The respective positions of each GPS satellite at the current time are then calculated by orbit information of each GPS satellite (hereinafter, referred to as ephemeris), thereby positioning the current position using the positions of each GPS satellite on the orbit and the above-mentioned pseudo range. This positioning allows obtainment of positioning positions defined by a latitude, a longitude, and an altitude, for example.
As mentioned above, times are necessary for positioning using GPS. However, it takes large costs and power consumption to provide a clock to measure correct time and keep the correct time.
With respect to this, a technique to reduce initial positioning time by calculating observable GPS satellites based on time information obtained from a RTC (Real Time Clock) or time information obtained from standard radio waves when the GPS receiver is turned on has been proposed (e.g., JP-A-2002-90441 (FIG. 2 and the like)).
However, there is a problem that since the RTC calculates the current time by counting clock counts after turning off the GPS at the last time, the accuracy of the RTC times used to calculate observable GPS satellites is sufficient, but that used to position may be insufficient. In addition, in order to receive standard radio waves, for example, an antenna and hardware different from a communication apparatus for a cellular phone must be provided separately.